All-liquid electrorheological effect

ABSTRACT

An apparatus for generating a giant electrorheological (ER) effect includes an upper high voltage electrode and a lower high voltage electrode, the upper high voltage electrode and the lower high voltage electrode each covered with a water-absorbing material and have water absorbed thereon. The apparatus also includes a fluid channel formed by layers and positioned in a gap between the upper high voltage electrode and the lower high voltage electrode; a pressure sensor positioned at one of the high voltage electrodes; a pump to flow silicon oil through the fluid channel; and a high voltage source configured to apply a voltage to the upper high voltage electrode. A method for generating a ER utilizes the apparatus.

BACKGROUND

Electric dipole, i.e. a positive charge and a negative charge separated by small distance, is probably the most common form of electrical entity in the world. Electric dipole moment is the measure of the separation of the positive and negative charges in a system or a measure of the charge system's overall polarity. Molecules with permanent molecular dipoles are denoted as polar molecules. For example, water molecules have a dipole moment of 2.7 D and urea molecules have a larger dipole moment of 4.6 D.

Molecular dipoles are typically randomly orientated or oppositely paired with the result that no net long range electric field is produced. However, molecular dipoles can be made to perform differently, and research into the ordering structures and thermodynamic properties of water molecules in the presence of different nano-confinements is varied and extensive. For example, molecular filaments have been observed in water-carbon nanotube (CNT) systems, and bilayer ice has been formed at 300K when water molecules are confined between nanoscale hydrophobic plates. Likewise, external application of an electric field can drive polar (hydrophilic) molecules into the non-polar (hydrophobic) phase. Due to the confinement exerted by the non-polar phase, the invading polar molecules tend to form some ordered structures, leading to a dramatic change in the rheological properties of the system.

Molecular dipoles can be made to align at contact regions of purpose fabricated nanoparticles under a moderate electric field, which results in large adhesion forces between the nanoparticles. This phenomenon, e.g. molecular dipole filament formation, was identified as the microscopic mechanism of, and is generally known as, the giant electrorheological (GER) effect. GER effect has been demonstrated in the formation of aligned molecular dipole filaments under the confinement effect of silicone oil chains.

In the context of the GER effect, urea molecules can penetrate a silicone oil layer with a thickness of several nanometers to form filament structures. However, the large energy barrier σ that must be overcome, results in urea filaments of a limited length. Urea coated BaTi(C₂O₄)₂ nanoparticles in silicone oil suspension exhibit a GER effect with the yield stress one order of magnitude larger than that of traditional ER fluids. Moreover, yield stress of GER fluids depends linearly on the electric field, which is quite different from the quadratic dependence shown by the ER fluids of the present disclosure and the tradition ER fluids. These characteristic yield stresses are accounted for by the urea dipole filaments formed in the contact region between two neighboring nanoparticles. Molecular dynamic (MD) simulation reveals that under the application of the electric field, urea molecular filaments are formed by penetrating the silicone oil layer from two sides, with the molecular dipoles predominantly aligned along the direction of the applied field. This ordered structure maximizes the number of hydrogen bonds formed between the invading urea molecules and minimizes the dipole field interaction energy to overcome the unfavorable energy barrier σ. The confinement effect on the urea filaments is offered by the silicone oil chains through the repulsive interaction between the silicone oil methyl groups and certain atoms in the urea molecules. Two sides of the gap are bridged by the filaments and a large attractive interaction arises leading to the GER effect. MD simulations using various gap sizes were conducted to determine the possibility of finding a urea molecule at the center of the gaps with different gap sizes. At 9 nm the possibility decreased to nearly zero. Thus, a physical, molecular bridge should be observed only at a smaller gap size. BRIEF

SUMMARY

The instant subject matter is directed to an apparatus for generating an electrorheological (ER) effect comprising an upper voltage electrode and a lower voltage electrode, the upper voltage electrode and the lower voltage electrode each covered with a water-absorbing material and have water absorbed thereon; a fluid channel formed by layers and positioned in a gap between the upper voltage electrode and the lower voltage electrode; a pressure sensor positioned at one of the voltage electrodes; a pump to flow silicon oil through the fluid channel; and a voltage source configured to apply a voltage to the upper high voltage electrode.

In another embodiment, the instant subject matter is direct to methods for generating an electrorheological (ER) effect comprising: providing an apparatus, the apparatus comprising: an upper voltage electrode and a lower voltage electrode, the upper voltage electrode and the lower voltage electrode each covered with a water-absorbing material and have water absorbed thereon; a fluid channel formed by layers and positioned in a gap between the upper voltage electrode and the lower voltage electrode; a pressure sensor positioned at one of the voltage electrodes; a pump; and a voltage source configured to apply a voltage to the upper voltage electrode and the lower voltage electrode; flowing silicone oil through the fluid channel using the pump; and applying voltage to the upper voltage electrode, thereby creating an electric field and generating the ER effect.

In yet another embodiment, the instant subject matter is directed to an all-liquid electrorheological (ER) fluid comprising: a mixture of about 85-95 wt % silicon oil and about 5-15 wt % water, wherein the silicon oil and water are uniformly mixed; and the mixture exhibits electrorheological effects when an outside voltage is applied to the mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a photograph of finished sample including high voltage electrodes and a fluid channel.

FIG. 2 depicts the structure of polymethylmethacrylate (PMMA) layers for laser engraving.

FIG. 3 illustrates an experiment setup system.

FIG. 4 depicts the pressure measurement before the application of high voltage.

FIG. 5 depicts the pressure measurement after the application of high voltage.

FIG. 6 depicts the introduction of an effective surface charge density.

FIG. 7 depicts the measured increment in pressure difference of water-silicone oil systems with a 0.5 mm gap.

FIG. 8 depicts the measured increment in pressure difference of water-silicone oil systems with a 0.75 mm gap.

FIG. 9 depicts the measured increment in pressure difference of water-silicone oil systems with a 1 mm gap

FIG. 10 depicts the measured increment in pressure difference of water-decane system with a 1 mm gap.

FIG. 11 depicts the measured ΔP″ as a function of the voltage applied with the quadratic fittings represented by the solid line.

FIG. 12 depicts the pressure difference plotted as the function of E².

FIG. 13 illustrates the experimental setup used for measuring GER effect.

FIG. 14 depicts the measured shear stress as a function of time (red curve), with the voltage pattern (black curve) plotted on the same graph.

FIG. 15 depicts the theoretical relationship between yield stress and strain.

FIG. 16 depicts a MD snapshot showing the initial configuration of the simulation in Example 8.

FIG. 17 MD snapshot showing the giant water molecular filaments with the presence of the electric field.

FIG. 18 depicts a sample of 5 μm gap was made of two water-penetrable AAO membranes sandwiched by two metal mesh electrodes.

FIG. 19 depicts the schematic structure of sample with gap size ˜5 μm.

FIG. 20 Measured ΔP″ as a function of the voltage applied for small gap size (5.15 μm) sample.

DETAILED DESCRIPTION

In the context of the instant subject matter, it has been found that the above shortcomings of known filament formation systems, namely finite filament length, can be overcome. For example, water can form macroscopic molecular filaments or files in silicone oil phase under an externally applied electric field. To this end, a microfluidic method for measuring the electrorheological (ER) effect originated from the field induced dipolar filament formation has been developed. The broad model includes a flow channel, two parallel plate electrodes coated with water-absorbing material(s), a syringe pump, a pressure sensor, and a high voltage source, e.g. electrodes. The amount of absorbed water on the electrodes and the volume rate of the carrier flow through the channel are carefully controlled, while the pressure difference across the channel is monitored. This method is more sensitive, accurate, and reliable than the known rotational ER meter, thereby expanding new possibilities in new material design and industrial applications.

Furthermore, recognition of molecular dipole filament formation as the microscopic mechanism of the giant electrorheological (GER) effect implies the possibility of the formation of macroscopic molecular dipole filaments inside a hydrophobic phase along the direction of the externally applied electric field. The electrorheological (ER) effect resulting from such structures can be defined as the molecular ER effect, which has been recently realized experimentally in water-silicone oil systems by using the microfluidic method.

The energy barrier for one urea molecule penetrating into silicone oil as a molecular filament can be larger than k_(B)T (k_(B) represents the Boltzmann Constant and T represents temperature). However, the dipole field interaction can only compensate for one k_(B)T per polar molecule, as the magnitude of the field goes to infinity. From Boltzmann statistics, it is possible to calculate the average fluctuating filament length

N

_(σ) by

$\begin{matrix} {{{\langle N\rangle}_{\sigma} = {1/\left( {{\exp \left( \frac{\sigma - {\Delta {\overset{\rightharpoonup}{p} \cdot \overset{\rightharpoonup}{E}}}}{k_{B}T} \right)} - 1} \right)}},} & (1) \end{matrix}$

where T denotes room temperature, N is the number of urea molecules in the filament, Δp is the difference of the dipole moment per molecule along the field direction between the filament state and bulk states, and E is the electric free energy in the field. The filaments are formed between the two substrates when the separation distances are on the order of 2

N

₉₄ l or slightly larger. l denotes the size of the dipolar molecules. Since −Δp·Ē→−k_(B)T is the limit of |E|→∞, and, in the case of urea/silicone oil, a σ>k_(B)T, determines the maximum saturation gap/saturation behavior of urea filaments. In relation to Equation (1), the critical investigation is whether any type of dipolar molecules exist having a σ˜k_(B)T or even a σ<k_(B)T so that the critical electric field at which σ−Δp·Ē=0 exists can be employed to produce macroscopic length filaments of these particular dipolar molecules.

Accordingly, a microfluidic method for measuring the electrorheological effect originating from the field induced dipolar filament formation that can be further defined as molecular ER effect is demonstrated using a channel between two parallel plate electrodes coated with water absorbing layers of material. In the model of the present subject matter, the volume rate of the carrier flow through the channel can be precisely controlled by a pump, in particular a syringe pump. However, other pumping mechanisms are also contemplated within the present apparatus and method. The pressure difference across the channel is monitored by a high sensitivity pressure sensor. An increment in the monitored pressure difference ΔP″ can be recorded if the field-induced molecular dipole filaments are formed. For water molecules, it is possible to form giant molecular filaments across gaps as large as ˜1 mm (macromolecular) in silicone oil phase. ΔP″ demonstrated quadratic dependence on the applied electric field in the presence of sufficient water supply. However, this gradually shifted to a linear dependency on the applied electrical field as the water supply was exhausted.

Using the apparatus and method of the present subject matter, the largest pressure increment measured at ˜5 kV/mm was 120 Pa. Interestingly, it is noted that this effect disappears when silicone oil is substituted by decane. Also, quadratic field dependence implies ΔP″ can reach the order of MPa when the gap size decreases to several microns. This is in sharp contrast to the linear effects observed when using urea molecules in silicon oil at distances of only a few nanometers.

Quadratic field dependence can also be modeled theoretically, thus verifying that the yield stress can reach the order of MPa when the gap size decreases to several microns. This corresponds to an electric energy density higher than that of any prior art GER materials. The method can be applied in may microfluidic-based devices, such as micro-clutches, micro-valves and micro-dampers, and facilitate the control of the fluidic logic systems. The mechanism can be applied to design functional materials, such as new GER materials, and biomaterials.

EXAMPLES Example 1 Fabrication of Microfluidic Measurement Apparatus

The apparatus is made of two major features: a pair of electrodes coated with water absorbing material(s) and a fluid channel (FIG. 1.). Voltage is applied directly on the electrodes and fluid flows through the channel between the electrodes. The pressure sensor is put near the electrodes to measure the pressure difference. Polymethylmethacrylate (PMMA) film is used as the walls of channel. The upper electrode is inserted into the PMMA wall. The lower electrode is made of a copper film and bonded with a spacer. The spacer is also made of PMMA film and the gap size (electrodes distance) is determined by the thickness of the spacer. In one aspect, the gap size is selected from 1 mm, 0.75 mm or 0.5 mm. It is noted that these gap sizes are much larger than the nanometer gaps found in the prior art systems.

PMMA 2-mm film was fabricated by the laser engraving machine (Universal Laser System) to form an electrode groove (FIG. 2). The six 3-mm diameter holes near the boundary facilitate mechanical sealing. The rectangular part (28 mm×10 mm) in layer 1 was cut away. The corresponding area in layer 2 was engraved to a depth around 200 μm. The square part in layer 2, the two 1 mm diameter small holes near the groove, and the bigger hole on the right of the groove were also cut through. The PMMA film in layer 3 served as the spacer to be put in the middle of the electrodes and control the channel height (0.5 mm, 0.75 mm or 1 mm). Layer 1 and layer 2 were bonded together with an acrylic solvent, with the gaps sealed, including the six holes for the screws and two small holes.

A 2 mm thick copper block (28 mm×10 mm) was machined, and another part of the electrode was made of copper film at a 1 mm thickness. The film corresponded in size to layer 3 (FIG. 2). Six holes were machined into the block to facilitate mechanical sealing. The PMMA spacer was attached to the copper film with 502 strong adhesive with pre-alignment. The copper block was inserted into the groove formed by layer 1 and layer 2 (FIG. 2) and affixed with epoxy.

Wire was connected to the electrodes through a square groove of the copper block and on the copper sides of the spacer layer part using tin solder, after the electrodes and the PMMA were bonded. The epoxy was then covered on the tin solder joint to fix the wire. A 0.9 mm stainless steel tube was inserted into the two small holes in the bonded PMMA as an inlet and connector for a pressure sensor. Plasticene was used to block the hole on the inner side. Epoxy was covered near the tube for sealing. Plastic hose was used to be the outlet. The finished upper and lower electrodes were combined thereby forming a fluid channel. Polydimethylsiloxane (PDMS) thin film was applied as a mechanical sealant. The PDMS thin film was fabricated with spin coater with a thickness of ˜50 μm.

Example 2 Measurement Protocol

The measurement experiment protocol requires the above measurement apparatus of Example 1, syringe pump, pressure sensor, multimeter, high voltage source, function generator and computer interface (FIG. 3). Polyester-based water-absorbing membrane (MEMBRA-CEL MC18×100 CLR) was immersed into deionized water for around 10 min. Then the membrane was cut and attached on the electrodes. A Philips PM5134 function generator triggered the DC high voltage supply with different generated signal (using a SPELLMAN SL300) which was connected with the upper electrode. High voltage signal was rectangular wave; duration was 40 s; and the duty cycle was 0.5. The signal was applied until breakdown occurred. The lower electrode was grounded. The pressure was measured with ultra-low pressure sensor (EdgeLight ELPR-5). LabVIEW acted as interface to collect the data. The silicone oil was pumped into the channel with a Harvard syringe pump. The outlet of the channel was open to the air and fixed at certain position.

Example 3 Pressure Measurement

The origin of the measured quantity ΔP″=ΔP′−ΔP is depicted in FIG. 4 and FIG. 5. Formation of dipolar molecular filaments provided resistance to the flow and additional energy was required to maintain the flow rate. This is reflected by the increment in the pressure difference which is nothing more than the increment of the input energy density. This additional part of energy density was used to (partially) break the dipolar filaments, and therefore should be equal to or smaller than the electric energy density of the filament structure.

Example 4 Estimate Model

Based on the physical picture established in Example 3, the following model estimates the effect. Free energy density equals the filaments' dipole-field interaction and dipole-dipole repulsive interaction. Minimizing free energy with respect to the number density of the filaments provides an equilibrium configuration and the corresponding energy density W that equals the measured increase in pressure difference ΔP″ based on constant-flow-rate measurement, wherein ΔP″ is the additional energy density required for breaking the filaments when the field is applied.

FIG. 6 shows the effective surface charge density at the two water oil interfaces separated at the gap distance. The charge density may be directly related to the number density of filament dipoles. To calculate energy density, using Gauss's law the displacement outside the oil layer will always be 0, which means that the presence of the electrodes and the dielectric constant of the water membrane have no effect on energy, and only the dielectric constant of oil counts.

The electric free energy at a field of E can be written as a function of the surface charge density σ:

$\begin{matrix} {{f(\sigma)} = {{\frac{1}{2{ɛɛ}_{0}}\sigma^{2}} - {\sigma \; E}}} & (1) \end{matrix}$

with a minimum at:

σ=εε₀E   (2)

And the resulting volume density of the free energy is:

$\begin{matrix} {{- \frac{1}{2}}{ɛɛ}_{0}E^{2}} & (3) \end{matrix}$

The measured change in pressure as a function of the applied electric field E can then be expressed as ΔP″=8.8E², and the coefficient can be obtained directly from experiment for comparison.

Example 5 Measurement and Observations

Samples with gap width of 0.5 mm, 0.75 mm and 1 mm were measured for water-silicone oil systems. FIGS. 6-9 show that, once high voltage is applied, the pressure difference increases immediately, and then falls back when the voltage was removed. Measurement continued until breakdown occurs (6 kV for 1 mm gap in water-silicone oil system). Without the water-absorbing membrane or with water-absorbing membrane but no water inside, no change in the pressure difference was observed. For water-decane system with 1 mm gap (FIG. 10) no change in pressure difference is observed, which implies no water molecular filament forms, because decane is more hydrophobic than silicon oil. The electric field can provide a finite driving energy which is smaller than the energy barrier for a water molecule to penetrate into decane, and thus the filament cannot be formed. However, in the case of silicon oil, this energy barrier is small and the filament can therefore be induced by the applied electric field. ΔP″ is measured as a function of the voltage applied. FIG. 11 shows the average of the results under high voltage. ΔP″ shows a quadratic voltage dependence at low voltage range. Under the same applied voltage, ΔP″ decreases as the gap size becomes larger. When the applied voltage is higher, ΔP″ gradually adopts linear voltage dependence possibly because quadratic dependence is a direct consequence of the free energy minimization, based on the assumption sufficient water molecules are used to form new filaments as required. As absorbed water gradually exhausts with voltage increases, the number of the filaments saturates. Therefore, the electric energy density of the filaments becomes linearly dependent on the voltage, leading to the linear dependence of ΔP″.

Example 6 Pressure Difference Plotted as the Function of E² and Observations

For all three samples of the water-silicone oil system, the voltage is normalized with the gap size, and average results measured in Example 5 are re-plotted as a function of E² (FIG. 12). All three curves collapse at low field range, indicating ΔP″ mainly depends on the applied electric field and has little relationship with the gap size. The change in pressure difference is proportional to E². From linear fitting, the coefficient is around 6, which corresponds to the value calculated from the previous electric energy, around 8.8. The measured coefficient is smaller than the calculated value, as expected.

The origin of ΔP″ cannot be due to the generation of a water layer between the silicone oil and the electrodes since that would introduce a slip boundary for the carrier flow and hence reduce the pressure difference. With the presence of a normal electric field, there can also be instability at the water-oil interface, owing to the dielectric constant contrast between the two media. However, the instability would allow water droplets to penetrate into silicone oil, decreasing the pressure difference since water has a lower viscosity than that of silicone oil. The comparative Experiments confirmed water must be responsible for the measured electrorheological effect resulting from dipolar molecular filaments.

Example 7 Rheometer Measurement

The effect observed in Example 6 is confirmed by rheometer measurement. Haake RS18 mm diameter circular rotating rheometer measures the molecular ER effect in the water-silicone oil system (FIG. 13). The voltage is applied through the two electrodes (the upper one rotates whereas the lower one is fixed). The gap is set to be 1 mm and filled with silicone oil. The electrode is covered with water-absorbed polyester-based membrane. 1 kV rectangular wave was applied. Once the electric field is applied, the shear stress increases quickly and can reach 80 Pa before falling back when the electric field is removed (FIG. 14).

In the context of the relationship between yield stress and strain, the consistency with the rheometer measurement is verified. Measured yield stress τ₀ is proportional to the electric energy density W. τ=aε for a linear strain-stress relation, where a is the constant and ε is the strain. It follows that

$W = {\frac{1}{2}a\; ɛ^{2}}$

and τ=2W/ε. The yield stress is the stress where the yield point ε₀ is reached, and therefore τ₀ ₌2W/ε₀ (FIG. 15). A yield stress of ˜88 Pa with a 1 kV/mm applied field is predicted, which is coincides with the preliminary measurement.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the instant subject matter as expressed in the appended claims.

Example 8 Molecular Dynamics Simulation

MD simulation was performed for a system composed of silicon oil (90 wt %) and water (10 wt %) uniformly mixed. See, FIG. 16 aMD snapshot showing the initial configuration of the simulation. Relatively uniform distribution of water molecules was realized by randomly inserting water molecules into the intervals of the silicone oil chains (omitted from the Figure to obtain a clear view). While the simulation was performed for a system composed of 90 wt % silicon oil and 10 wt % water, it is also contemplated that the amount of silicon oil can range from 85 wt % to 95 wt %; likewise the amount of water can range from 5 wt % to 15 wt %.

An electric field was applied along the direction shown by the arrow in FIG. 17. Through simulations, the dipoles of the water molecules in the filaments are mostly aligned along the electric field direction, and these filaments represent giant dipoles bridging the gap. This particular structure gives rise to the ER effect as observed in each of the examples.

This uniform water-oil mixture with low water fraction is a new type of all-liquid ER fluid because of the absence of a solid (particle) phase which is the key component in conventional ER and GER fluids. It should be noted that the system is capable of functioning with various fractions. Moreover, the device to realize the all-liquid ER effect can become extremely simple with two electrically insulated electrodes and two side walls as the channel. In this case, no water reservoirs would be needed.

Example 9 Fabrication of Microfluidic Measurement

Samples with small gap size of ˜5 μm were also examined using the apparatus described below. It was found ΔP″ increased to 2.2 kPa at an applied voltage of 1.6 kV. The sample of 5 μm gap was made of two water-penetrable AAO membranes sandwiched by two metal mesh electrodes, as shown in FIG. 18.

The apparatus s made of three major parts: metallic mesh electrodes with mesh size of 90 μm, AAO membrane and fluid channel. See FIGS. 18 and 19. The high voltage was applied directly on the metallic mesh electrodes. The fluid channel was formed by two AAO membranes (64 μm thickness), separated by spacers that were made by PS (polystyrene) microspheres and the gap size was determined by the size of the spacer. PS microspheres with diameter of 5.15 μm were used. A pressure sensor is put near the electrodes to measure the pressure difference. Epoxy sealing is used as the side walls of channel. Uniform pores with diameter of 100 nm on the AAO membrane were used for water supply. Ceramic (Resbond 920) was put inside the hole of PMMA film to serve as water reservoir. PMMA film (24 mm×10 mm) with thickness of 1 mm was fabricated by the laser engraving machine (Universal Laser System).

The upper PMMA film has three holes. The large hole with diameter of 6 mm was used to hold the ceramic and two small holes with diameter of 0.9 mm were used for connection of stainless steel tubes. The bottom PMMA film just had one large hole for water supply. Firstly, metal mesh and AAO were attached to the PMMA films with epoxy. Then two stainless steel tubes with diameter of 0.9 mm were inserted into the two small holes in the PMMA to be the fluid inlet and outlet. Epoxy was covered near the tube for sealing. PS microspheres with diameter 5.15 μm were used as spacer to separate two AAO membranes. At last two PMMA films were stick together by epoxy and ceramic was put inside the large hole to store water.

Example 10 Measurement Protocol

Measurement of this sample of Example 9 was performed similarly to that set forth in Example 2, except that the pressure difference was measured using the digital pressure meter (CWY50). In order to put forward the application of the all liquid ER effect into real application, parallel-plate channels with small gap sizes ˜5 μm to achieve a larger electric field induced pressure difference were used. As expected, an increment in the pressure difference was observed which is one order of magnitude larger than those for the large samples. The results, FIG. 20,(measured ΔP″ as a function of the voltage applied for small gap size(5.15 μm) sample). indicate a trend of quadratic dependence on the applied electric field in small field region, and a linear dependence as the applied voltage increases. Saturation behavior in the pressure difference was observed in large field region. Similar to previous cases, this observed pressure difference arises from the yield stress of the water dipolar filaments. The largest pressure increment measured is ˜2.2 kPa. Such effect is gone when silicone oil is substituted by decane. It has to be noted that the distance between the two electrodes is still large due to the sandwiched AAO membranes, compared with the channel size. The quadratic field dependence can imply that the yield stress can reach the order of MPa when the distance between the two electrodes decreases. This corresponds to an electric energy density higher than that of any GER materials. 

1. An apparatus for generating an electrorheological (ER) effect comprising: an upper voltage electrode and a lower voltage electrode, the upper voltage electrode and the lower voltage electrode each covered with a water-absorbing material and have water absorbed thereon; a fluid channel formed by layers and positioned in a gap between the upper voltage electrode and the lower voltage electrode; a pressure sensor positioned at one of the voltage electrodes; a pump to flow silicon oil through the fluid channel; and a voltage source configured to apply a voltage to the upper voltage electrode.
 2. The apparatus according to claim 1, wherein the gap between the upper voltage electrode and the lower voltage electrode is selected from 1 mm, 0.75 mm and 0.5 mm.
 3. The apparatus according to claim 1, wherein the pump is a syringe pump.
 4. The apparatus according to claim 1, wherein the water absorbing material is a polyester-based water absorbing membrane.
 5. The apparatus according to claim 1, wherein the voltage applied to the electrodes is between 1 kV and 6 kV, inclusive.
 6. The apparatus according to claim 1, further comprising a function generator configured to send a control signal to the voltage source.
 7. The apparatus according to claim 6, wherein the control signal is rectangular wave, duration of the signal is 40 seconds, and duty cycle is 0.5.
 8. A method for generating an electrorheological (ER) effect comprising: providing an apparatus, the apparatus comprising: an upper voltage electrode and a lower voltage electrode, the upper voltage electrode and the lower voltage electrode each covered with a water-absorbing material and have water absorbed thereon; a fluid channel formed by layers and positioned in a gap between the upper voltage electrode and the lower voltage electrode; a pressure sensor positioned at one of the voltage electrodes; a pump; and a voltage source configured to apply a voltage to the upper voltage electrode and the lower voltage electrode; flowing silicone oil through the fluid channel using the pump; and applying voltage to the upper voltage electrode, thereby creating an electric field and generating the ER effect.
 9. The method according to claim 8, wherein the gap between the upper voltage electrode and the lower voltage electrode is selected from 1 mm, 0.75 mm and 0.5 mm.
 10. The method according to claim 8, wherein the pump is a syringe pump.
 11. The method according to claim 8, wherein the water absorbing material is a polyester-based water absorbing membrane.
 12. The method according to claim 8, wherein the voltage applied to the electrodes is between 1 kV and 6 kV, inclusive.
 13. The method according to claim 8, further comprising a function generator configured to send a control signal to the voltage source.
 14. The method according to claim 8, wherein the control signal is rectangular wave, duration of the signal is 40 seconds, and duty cycle is 0.5.
 15. An all-liquid electrorheological (ER) fluid comprising: a mixture of about 85-95 wt % silicon oil and about 5-15 wt % water, wherein the silicon oil and water are uniformly mixed; and the mixture exhibits electrorheological effects when an outside voltage is applied to the mixture.
 16. The all-liquid ER fluid of claim 15, wherein the amount of silicon oil present is 90 wt % and the amount of water present is 90 wt %. 